Abstract:

In the case of a tool clamping device, in particular, for an operating
spindle of a processing machine, with an activation element (3) that can
be moved by a drive between a first position, in which the tool clamping
device is located in a clamped position, and a second position, in which
the tool clamping device is located in a released position, a sensor is
present in the form of at least one electrical component for detecting a
position of the activation element (3). The sensor is integrated
structurally into the tool clamping device and at least one section of
the activation element (3) directly forms a component of the one or more
electrical components, wherein its position influences an electrical
property of this component. The component can be an electrical switch
whose closed state depends on the position of the activation element (3)
in that at least one section of the activation element (3) is
electrically conductive and forms a contact electrode of the switch or a
part of such an electrode. However, it could also be a capacitor or an
inductor.

Claims:

1. Tool clamping device, especially for an operating spindle of a
processing machine, with an activation element that can be moved by a
drive between a first position in which the tool clamping device is
located in the clamped position and a second position in which the tool
clamping device is located in the released position, wherein for
detecting a position of the activation element a sensor is present in the
form of at least one electrical component that is integrated structurally
in the tool clamping device and in that at least one section of the
activation element directly forms a component of the one or more
electrical components and its position influences an electrical property
of this component.

2. Tool clamping device according to claim 1, wherein the component is an
electrical switch whose closing state depends on the position of the
activation element, in that at least one section of the activation
element is electrically conductive and forms a contact electrode of the
switch or a part of such an electrode.

3. Tool clamping device according to claim 2, wherein the activation
element is a hydraulically or pneumatically driven, linear moving piston,
in that a cylinder in which the piston moves forms a second contact
electrode of the switch or a part of such an electrode, and in that the
piston is supported so that it can slide in the cylinder and is
electrically high impedance or insulated in the radial direction.

4. Tool clamping device according to claim 3, wherein the piston forms an
electrical contact by a slip ring supported in an electrically
high-impedance or insulated way relative to the cylinder.

5. Tool clamping device according to claim 3, wherein the closing of the
switch is caused by a contact of the piston in one of its two end
positions with an electrically conductive stop that is formed on the
cylinder or is connected to this cylinder.

6. Tool clamping device according to claim 5, wherein, in its other end
position, the piston does not enter into electrical contact with the
cylinder.

7. Tool clamping device according to claim 4, wherein the closing of the
switch is caused by a contact of the piston in one of its two end
positions with an electrically conductive stop that is formed on the
cylinder or is connected to this cylinder.

8. Tool clamping device according to claim 7, wherein, in its other end
position, the piston does not enter into electrical contact with the
cylinder.

9. Tool clamping device according to claim 1, wherein the component is at
least one electrical capacitive element whose value depends on the
position of the activation element, in that at least one section of the
activation element is electrically conductive and forms at least one
electrode of the capacitor.

10. Tool clamping device according to claim 9, wherein the capacitive
element has two annular electrodes arranged coaxial to each other and
arranged one next to the other in the axial direction with external
terminals, in that a cylindrical or hollow-cylindrical, electrically
conductive section of the activation element covers the electrodes
coaxially and therefore forms with these electrodes a series circuit of
two capacitors, and in that the cross section of the section of the
activation element covering the electrodes varies in the axial direction
so that the values of the capacitances depend on the axial position of
the section.

11. Tool clamping device according to claim 10, wherein two series
circuits spaced apart from each other in the axial direction and each
having two capacitive elements are provided and in that the axial
variation in the cross section of the section of the activation element
covering the electrodes has a profile so that, in one of the two end
positions, the total capacitance of one series circuit has the greater
value and, in the other end position, the total capacitance of the other
series circuit has the greater value.

12. Tool clamping device according to claim 11, wherein the section of the
activation element is hollow cylindrical, in that one of the two total
capacitances is arranged on the inside and the other is arranged on the
outside of the hollow cylinder, and in that the variation of the cross
section takes place at two steps offset relative to each other in the
axial direction, wherein, of these steps, one is located on the inside
and the other is located on the outside.

13. Tool clamping device according to claim 12, wherein the two series
circuits of capacitive elements are arranged in two
parallel-to-each-other, non-diagonally-opposing arms of a bridge circuit,
and in that the variation in the cross section of the section of the
activation element covering the electrodes has a profile with respect to
the electrodes so that, for a movement of the activation element, the
capacitance value of one of the two series circuits increases and that of
the other decreases always at the same time.

14. Tool clamping device according to claim 11, wherein the two series
circuits of capacitive elements are arranged in two
parallel-to-each-other, non-diagonally-opposing arms of a bridge circuit,
and in that the variation in the cross section of the section of the
activation element covering the electrodes has a profile with respect to
the electrodes so that, for a movement of the activation element, the
capacitance value of one of the two series circuits increases and that of
the other decreases always at the same time.

15. Tool clamping device according to claim 1, wherein the component is at
least one electrical inductive element whose value depends on the
position of the activation element and in that at least one section of
the activation element is ferromagnetic and forms at least one magnetic
core of the inductive element.

16. Tool clamping device according to claim 15, wherein the inductive
element has the shape of a cylindrical coil, in that a cylindrical or
hollow-cylindrical, ferromagnetic section of the activation element
covers the coil coaxially, and in that the cross section of the section
of the activation element covering the coil varies in the axial
direction, so that the inductance value depends on the axial position of
the section.

17. Tool clamping device according to claim 16, wherein two inductive
elements are provided in the form of cylindrical coils that are spaced
apart from each other in the axial direction and in that the axial
variation in the cross section of the section of the activation element
covering the coils has a profile so that, in one of the two end
positions, the inductance of the one coil and, in the other end position,
the inductance of the other coil has the greater value.

18. Tool clamping device according to claim 17, wherein the section of the
activation element is hollow cylindrical, in that one of the two coils is
arranged on the inside and the other is arranged on the outside of the
hollow cylinder, and in that the variation in the cross section takes
place at two steps that are offset from each other in the axial direction
and of which one is located on the inside and the other is located on the
outside.

19. Tool clamping device according to claim 18, wherein the two inductors
are arranged in two parallel-to-each-other, non-diagonally-opposing arms
of a bridge circuit, and in that the variation in the cross section of
the section of the activation element covering the electrodes has a
profile with respect to the two coils so that, for a movement of the
activation element, one of the two inductances increases and the other
decreases always at the same time.

20. Tool clamping device according to claim 17, wherein the two inductors
are arranged in two parallel-to-each-other, non-diagonally-opposing arms
of a bridge circuit, and in that the variation in the cross section of
the section of the activation element covering the electrodes has a
profile with respect to the two coils so that, for a movement of the
activation element, one of the two inductances increases and the other
decreases always at the same time.

[0003]DE 100 43 006 C1 teaches a device. Here an assembly of plate springs
provides a tensile force for clamping a tool or tool holder in a tool
mount through a set of collet chuck segments. The tool is released by an
axial displacement of a so-called drawbar on which, for this purpose, a
corresponding counter force must be applied. This counter force is
applied as a compressive force by means of a piston driven hydraulically
or pneumatically onto the end face of the drawbar facing away from the
tool mount. The piston is therefore designated as a tool release piston.
As an alternative, in DE 10 2004 051 031 B3, the use of an electric drive
was also proposed for displacing the tool release piston.

[0004]For clamping a tool, the tool release piston is moved from its
active position, in which it deflects the drawbar against the force of
the plate-spring assembly and therefore sets the tool clamping device in
the released position, into its inactive position, in which it is not in
contact with the drawbar. Because the drawbar rotates during the
operation of the operating spindle whose component is the tool clamping
device, while the tool release piston stands still, when the operating
spindle starts up, there is no longer contact between the drawbar and the
tool release piston. In order to guarantee this, after the issuing of the
corresponding command by the control unit of the processing machine, a
specified time span must elapse before the operating spindle can be
started. This time span can last, for a maximum, during the disengagement
movement of the tool release piston from the drawbar. This means a
certain delay in the work cycle of the processing machine.

[0005]In the case of an incorrect function in the form of a disengagement
movement of the tool release piston from the drawbar that is too slow or
incomplete due to incorrect pressure ratios in the case of a hydraulic or
pneumatic drive of the piston or due to an incorrect cycle control by the
control unit of the processing machine, the operating spindle may start
even though the tool release piston is still in contact with the drawbar.
In the extreme case, this can lead to frictional fusing of the tool
release piston with the drawbar at the respective pressure contact faces,
i.e., to damage to the tool clamping device.

SUMMARY OF THE INVENTION

[0006]In consideration of this state of the art, the problem of the
present invention is to improve the functional reliability of a tool
clamping device according to the invention and to accelerate the cycle of
a tool exchange.

[0007]This problem is solved according to the invention by a tool clamping
device as set forth in the claims. Advantageous configurations of the
invention are specified in the subordinate claims.

[0008]According to the invention, the position of the tool release piston
is detected by a sensor that is integrated structurally into the tool
clamping device. This measure solves the problem that a use of readily
available position sensors for hydraulic or pneumatic cylinders is
decisively prevented by the special spatial conditions of a tool clamping
device specified for installation in an operating spindle, such as, in
particular, the presence of a rotating lubricant feed. The function
further provided according to the invention of the tool release piston as
a direct component of an electrical component guarantees, on one hand,
the functional reliability of the position detection and simplifies, on
the other hand, the construction of the sensor. Advantageous effects of
the invention include a reduction in unplanned spindle standstill times
or a reliable cycle sequence during the tool exchange. In addition,
through the feedback of the piston position, sequence optimizations are
possible that can lead to a time gain for each completed part.

[0009]A first embodiment of the invention consists of an electrical switch
in which the tool release piston closes an electrical contact through a
mechanical contact with an end-face contact and here acts as a contact
electrode or at least as part of such a contact electrode, that is,
conducts the current flowing in the case of a closed switch.

[0010]A second embodiment of the invention consists of an electrical
capacitive element in which a section of the tool release piston acts as
a capacitor electrode due to its metallic conductivity. Through a
cross-sectional change of the above section along the movement direction
of the piston, a dependency of the electrode distance and thus of the
capacitance value on the piston position can be achieved.

[0011]A third embodiment of the invention consists of an electrical
inductive element in which a section of the tool release piston made, in
this case, from ferromagnetic material, acts as a magnetic core. Through
a cross-sectional change of the above section along the movement
direction of the piston, a dependency of the distance between a coil and
the magnetic core allocated to this coil and thus of the inductance value
on the piston position can be achieved.

BRIEF DESCRIPTION OF THE FIGURES

[0012]The following description of embodiments with reference to the
drawings discloses additional details and advantages of the invention.
Shown in these drawings are:

[0013]FIG. 1, a longitudinal section of a part of a tool clamping device
in the clamped position,

[0014]FIG. 2, a longitudinal section of the part of FIG. 1 in the released
position,

[0015]FIG. 3, a longitudinal section of a part of a tool clamping device
equipped with a first embodiment of the invention in the clamped
position,

[0016]FIG. 4, a longitudinal section of the part of FIG. 3 in the released
position,

[0017]FIG. 5, detail X from FIG. 3 in enlarged representation,

[0018]FIG. 6, an evaluation circuit for the first embodiment shown in
FIGS. 3-5,

[0019]FIG. 7, a longitudinal section of a part of a tool clamping device
equipped with a second embodiment of the invention in the clamped
position,

[0020]FIG. 8, an evaluation circuit for the second embodiment shown in
FIG. 6 [sic; 7], and

[0021]FIG. 9, a longitudinal section of a part of a tool clamping device
equipped with a third embodiment of the invention in the clamped
position.

DETAILED DESCRIPTION OF THE INVENTION

[0022]Initially, the basic function of a tool clamping device of the type
forming the basis here with reference to FIGS. 1 and 2, each showing the
part of such a device that is relevant for the present invention, shall
be briefly explained with respect to the activation of the release of the
tool from the tool mount. In the clamped position shown in FIG. 1, a
not-shown plate-spring assembly exerts a force F on a drawbar 1 that is
supported so that it can move and of which only the rear end facing away
from the similarly not-shown tool mount is to be seen in FIGS. 1 and 2
and holds the drawbar 1 in the position shown in FIG. 1. A mechanical
coupling between the drawbar 1 and the collet chuck segments of the tool
holder provides, in this position, for the clamping of a tool or tool
holder in the tool mount.

[0023]Axially adjacent to the end of the drawbar 1, there is a hydraulic
cylinder 2 with an annular inner space in which an annular piston 3, the
so-called tool release piston 3, is mounted so that it can move. The tool
release piston 3 is located in the clamped position according to FIG. 1
on the rear end of its movement range and contacts a stop 5 with its rear
end face 4. A front chamber 6 in the hydraulic cylinder 2 is filled, in
this case, with a pressurized fluid and presses the tool release piston 3
against the stop 5. The tool release piston 3 has a
hollow-cylinder-shaped section 7 that extends in the direction of the
drawbar 1 and that has, however, in this position of the tool release
piston 3, a distance from the rear end of the drawbar 1.

[0024]In the released position shown in FIG. 2, the tool release piston 3
is shifted relative to the clamped position of FIG. 1 in the axial
direction against the drawbar 1 by filling a rear chamber 8 in the
hydraulic cylinder 2 with a pressurized fluid, so that the front end of
the hollow cylindrical section 7 of the tool release piston 3 is in
contact with the rear end of the drawbar 1 and exerts a counter force on
the drawbar 1, wherein this counter force is directed against the force F
of the plate spring assembly. This drawbar is here shifted with respect
to the clamped position of FIG. 1 in the axial direction, that is, in the
representation of FIGS. 1 and 2, toward the left in the direction of the
tool holder, wherein the collet chuck segments release the tool and the
tool can be exchanged.

[0025]Because the drawbar 1 rotates during the operation of the operating
spindle but the tool release piston 3, however, stands still, it is of
interest to monitor the position of the tool release piston 3, in
particular, to determine whether or not it is located in the end position
according to FIG. 1 and thus at a distance from the drawbar 1. Reaching
the above end position by the tool release piston 3 is a reliable
indicator of the presence of the clamped state in which the operating
spindle can be restarted again after a tool exchange.

[0026]A first embodiment of a tool clamping device according to the
invention with a sensor for detecting the position of the tool release
piston is explained below with reference to FIGS. 3 to 5. Here, FIG. 3
shows, like FIG. 1 before, a cutout from a tool clamping device of the
type forming the basis here. However, relative to FIG. 1, a few
components, among these, in particular, the drawbar 1, are left out and
therefore the remaining components are shown in detail. According to the
invention, the tool release piston 3 functions as a component of an
electrical switch, as described below. For forming this switch, the rear
wall 9 of the cylinder 2 is contacted electrically and guided to an
external connection terminal A. The rear wall 9 of the cylinder 2 is
here, in contrast to the schematic diagram of FIG. 1, a part that is
separate from the side wall 10, because the cylinder 2 must be open on
one side for inserting the tool release piston 3.

[0027]The tool release piston 3 is electrically contacted in the region of
its hollow-cylindrical section 7 projecting in the direction of the
drawbar, i.e., in the diagram of FIGS. 3 and 4, horizontally toward the
left by a metallic slip ring 11. This slip ring 11 is supported by means
of an elastic insulation ring 12, for example, an O-ring made from
insulating material, within a ring-shaped groove 13 that is formed in a
similarly hollow-cylinder-shaped guide body 14 lying coaxial to the
section 7 and guiding this section in its movement. The guide body 14 is
here, in contrast to the schematic diagram of FIG. 1, a separate part,
but it is connected to the rear wall 9. The slip ring 11 is electrically
insulated from the guide body 14 by the insulating ring 12. The
insulating ring 12 furthermore exerts a radially outward directed force
onto the slip ring 11 and presses it from the inside against the section
7 of the tool release piston 3. From the slip ring 11, an insulated line
15 leads to an external connection terminal B. For illustration, the
cutout characterized in FIG. 3 with the letter X is shown enlarged in
FIG. 5.

[0028]The mechanical contact of the tool release piston 3 with the rear
wall 9 of the cylinder 2 in the end position shown in FIG. 3
simultaneously creates an electrical contact between these two metallic,
conductive components, wherein, overall, an electrical connection between
the connection terminals A and B is produced by means of the
hollow-cylindrical section 7, the slip ring 11, and the line 15, if the
tool release piston 3 is located in the right end position shown in FIG.
3 in which it reliably has no mechanical contact with the drawbar and
consequently the tool clamping device must be located in the clamped
state. As soon as the tool release piston 3 is distanced from the rear
wall 9 of the cylinder 2, the electrical contact and thus the conductive
connection between the connection terminals A and B is broken.

[0029]Thus, a position sensor is present in the form of an electrical
switch whose closed state reliably indicates the right end position of
the tool release piston 3 and thus the clamped position of the tool
clamping device. The special reliability of this indication touches upon
the fact that the tool release piston 3 forms an integral component of
the switch in which it directly represents a contact electrode of the
switch and conducts current when the switch is closed.

[0030]A prerequisite for the function of the sensor is that the electrical
contact between the tool release piston 3 and the cylinder 2 can be
closed only at the rear wall 9 of the cylinder 2. In the case of a
hydraulic or pneumatic drive of the annular or hollow-cylindrical-shaped
tool release piston 3, several elastic sealing rings 16 are present on
its inner and outer lateral surfaces in order to create the seal
necessary for the function of the drive. Such a seal is required both
between the two annular chambers 6 (FIGS. 3) and 8 (FIG. 4) defined by
the position of the tool release piston 3 within the cylinder 2 and also
in the region of the hollow-cylindrical section 7 between the front
annular chamber 6 and the surroundings. Another function of the sealing
rings consists in the sliding support of the tool release piston 3 in the
cylinder 2.

[0031]The sealing rings 16 are made from insulating material and thus
simultaneously create electrical insulation of the tool release piston 3
relative to the cylinder 2 and also relative to the guide body 14 that is
connected mechanically and therefore also electrically to the rear wall 9
of the cylinder 2. In addition to the sealing rings 16 required for the
function of the drive, that is, for the sealing, additional insulation
rings can be provided at suitable positions, for example, in the front
region of the hollow-cylindrical section 7 and the guide body 14, in
order to reliably prevent electrically conductive contact of the section
7 in the region of its inner and outer lateral surfaces with the cylinder
2. It is understood that the medium for driving the tool release piston 3
must be non-conductive in each case in this embodiment of the invention.

[0032]FIG. 4 shows the section from a tool clamping device from FIG. 3
with the tool release piston 3 in its other end position in which it is
shifted as far as possible to the front, i.e., toward the left in the
representation of FIGS. 3 and 4, and in which the hollow-cylindrical
section 7 deflects the not-shown drawbar for releasing the tool clamping
device. This position of the tool release piston 3 corresponds to the
released position shown in FIG. 2. As is visible from FIG. 4, the tool
release piston 3 does not touch the front wall 17 of the cylinder 2 in
this position, but instead it still has a certain distance from it, so
that, in this end position, in contrast to the other position, there is
no electrical contact between the tool release piston 3 and the cylinder
2 and consequently there is no conductive connection between the
connection terminals A and B. This is achieved by a mechanical stop that
limits the movement range of the drawbar 1 and thus also the tool release
piston 3 in the direction of the tool mount, i.e., toward the left in
FIG. 4. Through a suitable measure, such as, for example, the arrangement
of an insulating disk on the rear end of the drawbar, en electrical
contact between the tool release piston 3 and the drawbar must be
prevented if the drawbar is not supported on its side such that it is
electrically insulated from the cylinder 2.

[0033]As FIG. 6 shows, the evaluation of the switch position can be
performed in a simple way by connecting the input of a Schmitt trigger ST
and a pull-up resistor RP to the power-supply voltage U0 of the
evaluation electronics at the connection terminal B. The connection
terminal A is connected via the cylinder 2 to the whole, remaining
metallic tool clamping device and via the operating spindle in which the
tool clamping device is installed to the ground of the processing
machine. This means that, for an opened switch S, through the pull-up
resistor RP, the connection terminal B lies at the power-supply
voltage U0 of the evaluation electronics and for a closed switch S
at the ground of the processing machine, as long as the resistor RN
shown with dashed lines is not at ground, which will be discussed below.
Thus, the switch S does not float, but instead always switches to ground
when closed. Through the Schmitt trigger ST and its input capacitance,
the switch S is debounced and a digital output signal is provided that
can be fed to the machine controller for processing. Here, a level
converter P can also be provided after the Schmitt trigger ST.

[0034]Due to wear and contamination in the case of long-term operation,
the insulation effect of the sealing rings 16 and/or additional
insulation rings can degrade and a certain electrical conductivity
between the tool release piston 3 and the cylinder 2 can be produced,
even if the tool release piston 3 is not in contact with the rear wall 9
of the cylinder 2. Such conductivity is tolerable for the function of the
position sensor according to the invention if the connection between the
tool release piston 3 and the cylinder 2 is at least high impedance apart
from the contact position on the rear wall 9 of the cylinder 2, so that
reaching the contact position can still be reliably detected.

[0035]In the case of the evaluation circuit according to FIG. 6 with a
Schmitt trigger ST and a pull-up resistor RP to the power-supply
voltage U0 at the connection terminal B, this means that the
resistor RN resulting from incomplete insulation between the tool
release piston 3 and the cylinder 2 and shown with dashed lines in FIG. 6
between the connection terminal B and ground forms a voltage divider
together with the pull-up resistor RP and the connection terminal B
is thus not pulled by the pull-up resistor RP to the power-supply
voltage U0 apart from the contact position of the tool release
piston 3, i.e., for an open switch S, but instead lies at a lower voltage
determined by the divider ratio of RP and RN. This voltage must
still lie sufficiently far above the switching threshold of the Schmitt
trigger ST, so that this reliably switches over when the switch S is
closed. Whether a certain value of RN is sufficiently high impedance
depends, in the circuit according to FIG. 6, on the divider ratio of the
voltage divider formed by RP and RN.

[0036]FIG. 7 schematically shows a second embodiment of the invention,
wherein the partial view of a tool clamping device corresponds to that of
FIG. 3, that is, represents the clamped position with the tool release
piston 3 in its rear end position. As can be seen in FIG. 7, in the
region of the hollow cylindrical section 7 of the tool release piston 3
projecting in the direction of the drawbar, both within and also outside
the hollow cylinder, two annular capacitor electrodes 18 and 19 or 20 and
21 are arranged in the axial direction one next to the other and adjacent
to each other, wherein there is a specified spacing in the axial
direction between the two inner-lying electrodes 18 and 19 and the two
outer lying electrodes 20 and 21.

[0037]The capacitor electrodes 18 to 21 are arranged similar to the
insulation ring 12 of the first embodiment each in separately formed
annular grooves that are not shown for the sake of simplicity in FIG. 7
and are necessarily electrically insulated relative to the guide body 14.
As can be taken from FIG. 7, the hollow-cylindrical section 7 of the tool
release piston 3 covers all of the capacitor electrodes 18 to 21 and thus
forms a series circuit of two capacitors between the terminals D and E or
F and G, respectively, guided insulated to the outside, on one side, with
the two inner lying electrodes 18 and 19 and, on the other side, with the
two outer lying electrodes 20 and 21.

[0038]As FIG. 7 further shows, the hollow cylindrical section 7 has two
steps 22 and 23 offset relative to each other in the axial direction at
which the cross section changes. The step 22 is located farther behind,
i.e., in FIG. 7 to the right on the inside and the step 23 farther
forward, i.e., in FIG. 7 to the left on the outside of the section 7. In
this way, a greater thickness is produced between the two steps 22 and 23
than toward the right from the step 22 and toward the left from the step
23. The capacitor electrodes 18 to 21 are arranged relative to the steps
22 and 23 so that, for a displacement of the tool release piston in the
axial direction, the rear step 22 moves past the inner electrodes 18 and
19 and the front step 23 moves past the outer electrodes 20 and 21,
wherein the corresponding electrode spacings of the capacitors connected
in series and thus the capacitance values change.

[0039]Through the orientation of the two steps 22 and 23, a displacement
of the tool release piston 3 in the direction of the drawbar, that is,
the movement into the released position, causes, on one hand, an increase
in the electrode spacing and thus a decrease in the capacitance value of
the inner capacitors with the electrodes 18 and 19, and on the other
hand, a reduction in the electrode spacing and thus an increase in the
capacitance value of the outside capacitors with the electrodes 20 and
21. A displacement of the tool release piston 3 in the opposite direction
accordingly causes inverted capacitance-value changes.

[0040]Furthermore, the positions and also the axial spacing of the two
steps 22 and 23 and the positions and also the axial spacing of the
inside electrode pair 18, 19 and the outside electrode pair 20, 21 are
tuned to each other, so that, for a displacement of the tool release
piston 3 in the axial direction, the inside capacitance decreases with a
simultaneous increase in the outer capacitance and vice versa. In
addition, the mentioned spacings and positions are selected so that, in
each of the two end positions of the tool release piston 3, one of the
two electrode pairs 18, 19 or 20, 21 just reaches its minimum capacitance
and the other just reaches its maximum capacitance.

[0041]In FIG. 7, the previously described dimensioning rules are expressed
in that, in the rear end position of the tool release piston 3 shown
there, the rear step 22 on the inside of the section 7 is located just to
the right next to the inner electrode pair 18, 19 and the front step 23
on the outside of the section 7 is located just to the right next to the
outer electrode pair 20, 21, wherein the total capacitance of the inner
electrode pair 18, 19 between the terminals D and E just reaches its
maximum value and that of the outer electrode pair 20, 21 between the
terminals F and G just reaches its minimum value.

[0042]In the not-shown, front end position of the tool release piston 3,
the rear step 22 on the inside of the section 7 would be located just to
the left next to the inner electrode pair 18, 19 and the front step 23 on
the outside of the section 7 would be located just to the left next to
the outer electrode pair 20, 21, wherein the total capacitance of the
inner electrode pair 18, 19 between the terminals D and E would just
reach its minimum value and that of the outer electrode pair 20, 21
between the terminals F and G would just reach its maximum value. For
this purpose, it is to be noted that the displacement path of the tool
release piston 3 does not correspond to the total axial length of the
front chamber 6 because the tool release piston 3, as explained above,
does not contact the front wall 17 of the cylinder 2 in the front end
position.

[0043]Therefore, a position sensor is present in the form of a capacitor
arrangement with several capacitance values changing in opposite
directions, wherein a set of extreme values of the capacitance values
reliably indicates the presence of the rear end position of the tool
release piston 3 and thus the clamping position of the tool clamping
device. The special reliability of this indication involves the condition
that a section 7 of the tool release piston 3 forms an integral component
of the capacitance arrangement in the form of an electrode common to
several capacitors.

[0044]One special advantage of the arrangement of several capacitors
described above in comparison with an individual capacitor is the better
measurability of a capacitance change relative to large stray
capacitances caused by the metallic surroundings. Offset and interference
parameters therefore can be computationally eliminated to a certain
extent. FIG. 8 shows a measurement circuit suitable for evaluation in the
form of a bridge circuit. Here, the inside and the outside series circuit
of capacitive elements are connected in parallel-to-each-other,
non-diagonally-opposing bridge arms, in order to achieve a maximum
possible effect on the diagonal voltage through simultaneous decrease and
increase in the equal-sized capacitance values C by the same magnitude
ΔC.

[0045]Because the section 7 of the tool release piston 3 active as a
capacitor electrode is metallic and conductive, there is a cross
connection between the two capacitive bridge arms through which two of
the capacitors are connected in parallel to each other and in series to
the bridge. The basic configuration of a half bridge, however, is
maintained. The connections D to G of the capacitors from FIG. 7 are
designated accordingly in FIG. 8. With the capacitors, two suitable
impedances ZX and ZY are connected together, in order to
obtain, overall, a bridge circuit.

[0046]Although the use of several capacitance values that can change in
opposite directions through a movement of the tool release piston 3 and
their connection together into a bridge appears especially advantageous,
a movement of the tool release piston 3 can also be basically detected
with reference to the measurement of a changing individual capacitance
value, i.e., only the capacitance formed by the electrodes 18, 19 or 20,
21 and the section 7 of the tool release piston 3, or two separate
measurements could be performed on two variable individual capacitors,
without these having to be connected together into a bridge.

[0047]FIG. 9 schematically shows a third embodiment of the invention,
wherein the partial view of a tool clamping device there similarly
corresponds to that of FIG. 3, that is, represents the clamped position
with the tool release piston 3 in its rear end position. As can be seen
in FIG. 9, in the region of the hollow cylindrical section 7 of the tool
release piston 3 projecting in the direction of the drawbar both within
and also outside the hollow cylinder, a coil 24 or 25 is present at a
specified mutual axial spacing.

[0048]The coils 24 and 25 are arranged just like the insulating ring 12 of
the first embodiment in separately formed annular grooves that are not
shown in FIG. 9 for the sake of simplicity. Through the use of highly
permeable material as the coil body, i.e., covering the mentioned grooves
with such material, penetration of the magnetic field into the
surrounding material of the guide body 14 can be largely prevented in
order to improve the intended measurement effect described below. As to
be taken from FIG. 9, the hollow-cylindrical section 7 of the tool
release piston 3 covers both coils 24 and 25 and forms a magnetic core
for each of these coils, which assumes that it is made from a
ferromagnetic material. The terminals of the coils are guided insulated
from the outside and designated with H and I or J and K.

[0049]As FIG. 9 further shows, the hollow-cylindrical section 7 has two
steps 22 and 23 offset relative to each other in the axial direction,
just as in the second embodiment, at which the cross section changes. The
step 22 is located farther toward the rear, i.e., in FIG. 9 to the right
on the inside and the step 23 farther toward the front, i.e., in FIG. 9
to the left on the outside of the section 7. In this way, a larger
thickness is produced between the two steps 22 and 23 than to the right
from the step 22 and to the left from the step 23. The coils 24 and 25
are arranged relative to the steps 22 and 23 so that, for a displacement
of the tool release piston 3 in the axial direction, the rear step 22
moves past the inside coil 24 and the front step 23 moves past the
outside coil 25, wherein the respective inductance values change.

[0050]Through the orientation of the two steps 22 and 23, a displacement
of the tool release piston 3 in the direction of the drawbar, that is,
the movement into the released position, causes, on one hand, an increase
in the spacing of the section 7 active as a magnetic core from the inside
coil 24 and thus a decrease in inductance, on the other hand, a reduction
in the spacing of the section 7 active as a magnetic core from the
outside coil 25 and thus an increase in inductance. A displacement of the
tool release piston 3 in the opposite direction causes correspondingly
inverted inductance-value changes.

[0051]Furthermore, the positions and also the axial spacing of the two
steps 22 and 23 and the positions and also the axial spacing of the
inside coil 24 and the outside coil 25 are tuned to each other so that,
for a displacement of the tool release piston 3 in the axial direction, a
decrease in the inside inductance with a simultaneous increase in the
outside inductance is generated and vice versa. In addition, the
mentioned spacings and positions are selected so that in each of the two
end positions of the tool release piston 3, one of the two coils 24 or 25
just reaches its minimum inductance and the other just reaches its
maximum inductance.

[0052]In FIG. 9, the dimensioning rules described above are expressed in
that, in the rear end position of the tool release piston 3 shown there,
the rear step 22 on the inside of the section 7 is located just to the
right next to the inner coil 24 and the front step 23 on the outside of
the section 7 is located just to the right next to the outer coil 25,
wherein the inductance of the inner coil 24 just reaches its maximum
value and that of the outer coil just reaches its minimum value.

[0053]In the not-shown front end position of the tool release piston 3,
the rear step 22 on the inside of the section 7 would be located just to
the left next to the inner coil 24 and the front step 23 on the outside
of the section 7 would be located just to the left next to the outer coil
25, wherein the inductance of the inner coil 24 would just reach its
minimum value and that of the outer coil 25 would just reach its maximum
value. For this purpose it is to be noted, in turn, that the displacement
path of the tool release piston 3 does not correspond to the total axial
length of the front chamber 6 because the tool release piston 3, as
mentioned before, does not contact the front wall 17 of the cylinder 2 in
the front end position.

[0054]Thus, a position sensor is present in the form of an inductor
arrangement with several inductors that can change in opposite
directions, wherein a set of extreme values of the inductors reliably
indicates the presence of the rear end position of the tool release
piston 3 and thus the clamped position of the tool clamping device. The
special reliability of this indication involves the condition that a
section 7 of the tool release piston 3 forms an integral component of the
inductor arrangement in the form of a magnetic core common to several
coils.

[0055]One special advantage of the previously described arrangement of
several coils in comparison with an individual coil is the better
measurability of an inductance change relative to large stray inductances
caused by the surroundings with ferromagnetic metal. Offset and
interference parameters can therefore be computationally eliminated to a
certain extent. For evaluation, in this embodiment, a measurement circuit
is similarly especially suitable in the form of a bridge circuit with a
circuit analogous to FIG. 8 of the inner and outer inductors in
parallel-to-each-other, non-diagonally-opposing bridge arms, in order to
achieve a maximum possible effect on the diagonal voltage through
simultaneous decrease and increase in equal-sized inductance values L by
an equal magnitude ΔL. Because the section 7 of the tool release
piston 3 acting as a magnetic core covers both coils 24 and 25, in this
case there is a counter-inductance between the two inductive bridge arms,
which is optionally to be taken into consideration in the design of the
circuit. However, operation is also possible here basically with only a
single measurement inductor or separate measurements could be performed
on two variable individual inductors.

[0056]From the preceding description, for someone skilled in the art, a
series of possible variants emerges for realizing the invention. For
example, in the first embodiment, the activation element could form a
contact instead of by means of a slip ring, also by means of a flexible
and loop-shaped guided line at a fixed point, as long as for such a line
a sufficient movement space can be made available for the movement.
Furthermore, on the rear wall of the cylinder or on the end face of the
tool release piston, a spring body could be attached, in order to define
a certain contact point. In this case, the cylinder or the tool release
piston would no longer provide the contact surface itself, but would
still be a component of the electrode. In the second and third
embodiment, in principle, a single capacitor or inductor would be
sufficient for detecting the rear end position of interest in the
activation element, although then, due to stray capacitance or inductance
that is present, a significantly larger expenditure with respect to
circuitry would be required. Such modifications and comparable
modifications lie at the discretion of someone skilled in the art and
should be covered by the protection of the claims.

[0057]Accordingly, although specific embodiments of the invention have
been disclosed, those having ordinary skill in the art will understand
that changes can be made to the specific embodiments without departing
from the spirit and scope of the invention. The scope of the invention is
not to be restricted, therefore, to the specific embodiments, and it is
intended that the appended claims cover any and all such applications,
modifications, and embodiments within the scope of the present invention.